(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land based or satellite stations. Omnidirectional antenna systems are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land based and satellite stations. In satellite communication applications it is desirable to provide a unidirectional antenna system that is compact yet characterized by a wideband width and a good front-to-back ratio, i.e., the ratio of overhead power to backside power, such that its pattern ideally only occupies the upper hemisphere.
Some prior art omnidirectional antenna systems use an end fed quadrifilar helix antenna for satellite communication and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth and collectively their performance can be dependent upon antenna position relative to a ground plane. The dipole antenna tends to have no front-to-back ratio which can cause total pattern cancellation when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations.
The following patents disclose helical antennas that exhibit some, but not all, the previously described desirable characteristics:
U.S. Pat. No. 3,599,220 (1971) Demsey PA1 U.S. Pat. No. 3,623,113 (1971) Faigen et al. PA1 U.S. Pat. No. 4,243,993 (1981) Lamberly et al. PA1 U.S. Pat. No. 4,644,366 (1987) Scholz PA1 U.S. Pat. No. 5,053,786 (1991) Silverman et al. PA1 U.S. Pat. No. 5,134,422 (1992) Auriol PA1 U.S. Pat. No. 5,170,176 (1992) Yasunaga et al. PA1 U.S. Pat. No. 5,343,173 (1994) Balodis et al. PA1 U.S. Pat. No. 5,594,461 (1997) O'Neil, Jr. PA1 U.S. Pat. No. 5,635,945 (1997) McConnell PA1 U.S. Pat. No. 5,257,032 (1993) Diamond et al. PA1 1. a pitch angle of the elements on the helix cylindrical surface from 50.degree. down to roughly 20.degree.; PA1 2. elements that are at least roughly 3/4 wavelengths long; and PA1 3. a "cut-in" frequency roughly corresponding to when a turn of an element on the helix cylinder is 1/2 wavelength long. (This dependence changes some with pitch angle. Above the "cut-in" frequency, the helix has an approximately flat VSWR, around 2:1 or less about the Z.sub.o value of the antenna, and thus the antenna is broadband impedancewise above "cut-in".)
U.S. Pat. No. 3,599,220 to Dempsey discloses a conical, spiral loop antenna comprising a plurality of pairs of spirally wound radiating arms. The radiating arms are wound in the shape of a cone and terminate at one end in a truncated portion. Impedance matching is provided between each of the pairs of radiating arms at the truncated end. A ground plane is provided for each frequency of operation; multiple ground planes are required for multiple frequencies. The primary purpose of this patent is to provide a compact antenna that is tunable. However, it appears that the antenna is generally tuned for a specific frequency.
U.S. Pat. No. 3,623,113 to Faigen et al. discloses a balanced, tunable, helical mono-pole antenna that operates independently of a ground plane. This antenna utilizes a centrally fed, multiple-turn, helical antenna with a single element. End winding shorting means in the form of "top hat"- or "can"-type housings tune the antenna by changing the active electrical length of the antenna. A feed loop is centrally disposed to the helical mono-pole antenna winding to provide a balanced input to the antenna. Although this antenna is compact and can be tuned through a wide bandwidth, it does not provide an omnidirectional radiation pattern.
U.S. Pat. No. 4,243,993 to Lamberty et al discloses a broad band antenna comprising center fed, spiral antenna arms arranged on planar and conical surfaces. Each antenna arm includes one or more choke elements that resonate at a predetermined operating frequency to eliminate or minimize undesired radiation and reception characteristics and provide sum and difference mode operations with both right-hand and left-hand circularly polarized radiation characteristics. Feeding an antenna as disclosed in the Lamberty et al patent with a phased sequence of signals produces a radiation pattern that exhibits a null along an antenna bore sight axis and a maximum field along a cone of revolution about the bore sight axis. Although this antenna has a broad bandwidth and provides circular polarization, it does not provide an omnidirectional radiation pattern.
U.S. Pat. No. 4,644,366 to Scholz discloses a miniature radio transceiver antenna formed as an inductor wrapped about a printed circuit card. A peripheral conductor on one side of the card provides distributed capacitance to the end of the antenna that cancels inductive effects and broadens bandwidth. A peripheral conductor on the opposite side of the card provides a capacitance to ground to tune the antenna to frequency. An unbalanced transmission line connects between one end of the antenna and a tap or feed point to provide impedance matching and tuning. This antenna has a limited bandwidth for a given connection point. Moreover it does not produce an omnidirectional radiation pattern.
U.S. Pat. No. 5,053,786 to Silverman et al. discloses a broad band directional antenna in which two contiguous conductive planar spirals are fed at their center. The antenna is positioned near a cavity to absorb rear lobes in order to improve the front-to-back ratio. Even with this improvement in the front-to-back ratio, the antenna provides a relatively narrow beam pattern having both horizontal and vertical polarization. Apparently, this antenna is designed to operate with a linearly polarized, high gain, narrow beam. Thus the antenna does not provide an omnidirectional radiation pattern or circular polarization. Moreover, by absorbing the rear lobes, the power transmitted into the reserve lobes is lost making the antenna less efficient in radiating during a transmitting mode.
U.S. Pat. No. 5,134,422 to Auriol discloses an antenna with helically wound, equally spaced, radiating elements disposed on a cylindrical surface. Antennas identified as prior art antennas in this reference include helically wound, end driven antenna elements. The other ends of the elements terminate as open circuits. These antennas provide circular polarization, an omnidirectional radiation pattern and a good front-to-back ratio. The Auriol patent is particularly directed to a structure that uses a conductive, meandering strip to connect the driven ends and establish various phase relationships and tuning. This antenna is designed to produce high quality circular polarization, an omnidirectional radiation pattern and a good front-to-back ratio, but only over a narrow frequency band.
In U.S. Pat. No. 5,170,176 (1992) to Yasunaga et al. a quadrifilar helix antenna includes four helix conductors wound around an axis in the same winding direction. Each helix conductor has a linear conductor which is parallel to its axis at either end or both ends of the helix conductor. The purpose of this structure is to reduce the effect of multipath fading due to sea-surface reflection in mobile satellite communications. Although this patent discloses an antenna that provides good front-to-back ratio, the transmission pattern from the antenna is also characterized by essentially forming two major lobes about 60.degree. from the forward direction so it is not truly omni-directional over a hemisphere.
U.S. Pat. No. 5,343,173 (1994) to Balodis et al. discloses a phase shifting network and antenna including a series of helical antenna elements with a phase shifting network defining transmission paths between a radio connection terminal and the antenna elements. Each transmission path phase shifts the signal relative to an adjacent path pairs that are progressively joined at combiner nodes of equal power division by shunt connection line segments.
U.S. Pat. No. 5,594,461 (1997) to O'Neill, Jr. discloses a low loss quadrature matching network for a quadrifilar helix antenna. As in the above-identified Balodis et al. patent, the O'Neill, Jr. patent utilizes microstrip techniques to provide impedance matching in an antenna system.
U.S. Pat. No. 5,635,945 (1997) to McConnell et al. discloses a quadrifilar helix antenna with four conductive elements arranged to define two separate helically twisted loops, one differing slightly in electrical length from the other. The two separate helically twisted loops are connected to each other in a way as to provide impedance matching, electrical phasing, coupling and power distribution for the antenna. The antenna is fed at a tap point on one of the conductive elements determined by an impedance matching network which connects the antenna to a transmission line. Like to foregoing Balodis et al. and O'Neill, Jr. patents, this patent also utilizes microstrip techniques to feed and match through a partly balanced transmission line. As a result the resultant band width is narrow.
The following patent discloses a broadband antenna system:
This broadband antenna system includes a frequency-independent antenna coupled to the frequency-dependent antenna, specifically, a spiral antenna and a dipole antenna. In one embodiment the antenna system comprises a dipole or monopole coupled to the inner or outer termination points of a spiral antenna. The spiral antenna acts as a broadband transmission line matching section and adds electrical length to the monopole antenna. Thus, the spiral antenna is stated to minimize the negative effects typically associated with the removal of one of the elements of a stand alone dipole antenna to create a monopole antenna. It is believed that when the dipole antenna is added to the termination points of the spiral antenna, the resulting antenna system extends the low frequency capability of the spiral antenna for linear polarization. It is also felt that the spiral antenna adds electrical length to the dipole antenna and acts as a broadband transmission line matching section so that the spiral antenna enhances receiving capability by producing a maximum signal at the transmission lines. This patent discloses the combination of two types of antennas. However, the combination includes a spiral antenna and either a monopole or dipole antenna. It also appears that the antenna system is directional and not omni-directional over both a broad frequency band and over a hemispherical volume.
Thus there exists a family of quadrifilar helixes that are broadband impedance wise above a certain "cut-in" frequency, and thus are useful for wideband satellite communication (DAMA function of 240 to 320 MHz, other functions at 320 to 410 MHz). Typically these antennas have:
The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at "cut-in".
For pitch angles of approximately 30.degree. to 50.degree., good cardoid shaped patterns exist for satellite communications. Good circular polarization exists down to the horizon since the antenna is greater than 1.5 wavelengths long (2 elements constitute one array of the dual array, quadrifilar antenna) and is at least one turn. At the "cut-in" frequency, the lower pitch 17 angled helixes have sharper patterns. As frequency increases, patterns start to flatten overhead and spread out near the horizon. For a given satellite band to be covered, a tradeoff can be chosen on how sharp the pattern is allowed to be at the bottom of the band and how much it can be spread out by the time the top of the band is reached. This tradeoff is made by choosing where the band should start relative to the "cut-in" frequency and by choosing the pitch angle.
For optimum front-to-back ratio performance, the bottom of the band should start at the "cut-in" frequency. This is because for a given element thickness, backside radiation increases with frequency (the front-to-back ratio decreases with frequency). This decrease of front-to-back ratio with frequency limits the antenna immunity to multipath nulling effects.